1. Field of the Invention
This invention relates to a developer for developing electrostatically charged images in electrostatography including electrophotography, electrostatic recording, electrostatic printing and others and also to a developing method using the same. More particularly, this invention pertains to a developer for electrophotography, which is strongly and uniformly charged to positive charges and is used for visualizing negatively charged electrostatic images to give images of high quality in the direct or indirect electrophotographic developing process, and also to an electrostatographic developing method using the same.
2. Description of the Prior Art
A large number of electrophotographic processes have been known, as disclosed in U.S. Pat. No. 2,297,691 and others. Generally speaking, photoconductive materials are utilized in these processes, and the steps included therein comprise forming electrical latent images on photosensitive members by various means, then developing the latent images by using developing powders (frequently called as "toner"), transferring the toner images thus formed to a recording medium such as paper, as desired, and thereafter fixing the images by heating, pressure or solvent vapor to obtain copies. When the step of transferring the toner images is applied, it is a general practice to provide a step for removing residual toner on the photosensitive member.
The developing metods for visualizing electrical latent images by use of toners known in the art may include, for example, the magnetic brush method as disclosed in U.S. Pat. No. 2,874,063; the cascade developing method as disclosed in U.S. Pat. No. 2,618,552; the powder cloud method as disclosed in U.S. Pat. No. 2,221,776; the method using conductive magnetic toner as disclosed in U.S. Pat. No. 3,909,258; the method using various insulating magnetic toners as disclosed in Japanese Patent Publication No. 9475/1976; and others.
As the toner for dry development system to be applied for these developing methods, use has heretofore generally been made of fine powder of natural or synthetic resins having dyes or pigments dispersed therein. For example, a colorant is dispersed in a binder resin such as polystyrene, and the particles obtained by micropulverizing the resultant dispersion into sizes of about 1 to 30.mu. are used as the toner. As the magnetic toner, magnetic particles are further incorporated into the particles as mentioned above. In case of the system employing the so called two-component developer, the toner as mentioned above is used generally in mixture with carrier particles such as glass beads, iron particles and others.
In a series of developing methods of electrostatographic processes as mentioned above, wherein electrostatically charged images are developed with a toner charged to the polarity opposite to that of the images, it is very important that the toner should be charged to the opposite polarity evenly in order to give good and faithful developing characteristics. For this purpose, it is generally practiced to add a positive or negative charge controller in addition to the above components in order for the toner to be charged evenly. Among such charge controllers, positive charge controllers having satisfactory characteristics are very few, while there have been developed some negative charge controllers having good characteristics.
Positive charge controllers conventionally used in toners for dry development system, may include, for example, amino compounds, quaternary ammonium compounds and organic dyes, particularly basic dyes and salts thereof. More specifically, benzyldimethylhexadecylammonium chloride, decyl-trimethylammonium chloride, nigrosine base, nigrosin hydrochloride, safranine .gamma., crystal violet and others are known. Particularly, nigrosine base and nigrosine hydrochloride have been frequently used as positive charge controllers. These dyes are usually added to a thermoplastic resin together with a colorant in the course of formation of a toner to be dispersed in the resin while it is molten under heating, and the resultant resin mixture is micropulverized into fine particles, adjusted to suitable sizes, if desired, and then provided for use as a toner.
However, these dyes as charge controllers have complicated structures and do not have constant properties, thus being poor in stability. Also, decomposition or denaturation may occur through decomposition, mechanical collision and friction during kneading underheat or change in temperature and humidity conditions, to cause lowering in the charge controlling characteristic.
Accordingly, when development is carried out by use of a toner containing these dyes as charge controllers in a copying machine, the dyes may undergo decomposition or denaturation as the increase in number of copies to cause deterioration of the toner during continual use.
As another serious disadvantage, it is very difficult to disperse these dyes as charge controllers evenly into a thermoplastic resin, and their contents in toner particles obtained by pulverization are not constant to result in different amounts of triboelectric charges among the toner particles. For this reason, in the prior art, various methods have been practiced in order to disperse these dyes more evenly into a resin. For example, a basic nigrosine dye is formed into a salt with a higher fatty acid for improvement of compatibility with a thermoplastic resin. In this case, however, unreacted fatty acid or decomposed product of the salt will be exposed on the toner surfaces to contaminate carriers or toner carrying member and also cause lowering in free flowing properly of the toner, fog and lowering in image density. Alternatively, for improvement in dispersibility of these dyes into a resin, there is also employed a method in which dye powders and resin powders are previously mechanically pulverized and mixed before fusion kneading. This method is not competent enough to overcome the original poor dispersibility, and evenness of charging satisfactory in practical application has not yet been obtained.
Most of dyes for positive charge controlling are hydrophilic and therefore, due to poor dispersibility of these dyes into a resin, the dyes are exposed on the toner surfaces when pulverized after fusion kneading. When the toner is used under highly humid conditions, a drawback that no image of good quality can be obtained, is encountered because of hydrophilic nature of the dye.
Thus, when a dye having positive charge controlling characteristic of the prior art is used in a toner, variances in amount of the charges generated on the toner particle surfaces through friction among toner particles, between toners and carriers or between toners and toner carrying member such as sleeve, will occur, whereby various inconveniences are caused, such as development fog, toner scattering, carrier contamination, etc. These difficulties become marked when a large number of copying cycles were carried out continuously, giving the results essentially unsuitable for a copying machine.
Also, when the toner containing a positive charge controlling dye is used, the transfer efficiency of the toner image is markedly lowered under highly humid conditions to be unsuitable for practical use. Even under normal temperature and humidity, when the toner is stored for a long time, due to instability of the positive charge controlling dye employed, toner particles may frequently agglomerate to become useless.
As a method for obtaining a positive charge controlling developer, there is a proposal as disclosed in Japanese Patent Publication No. 22447/1978. This is a method in which metal oxide particles treated with aminosilanes are incorporated as a constituent of the developer. However, as the result of my investigation on this method, when developers were obtained according to the examples disclosed in the Japanese Patent by applying treatments on metal oxides such as colloidal silica, alumina, titanium dioxide, zinc oxide, iron oxide, .gamma.-ferrite, magnesium oxide, etc., no developer exhibiting satisfactory characteristics in practical application could be obtained in any of the combinations, but such a developer was found to have some drawbacks.
Thus, most of the developers thus prepared cannot maintain the preferable characteristics for faithful reproduction of latent images. Although desirable performance may be exhibited at the initial stage, it cannot be maintained in continuous use for a long term to make the developer useless. More specifically, fog is formed with scattering of the toner around the edges in copying of line images, and the image density is also lowered.
As other drawbacks, when developing and transfer are conducted under the conditions of higher temperature and humidity, and lower temperature and humidity, lowering in image density, scattering of line images, drop-off of images, fog, etc., are encountered. These phenomena are observed in both developing step and transfer step.
As still another drawback, the developer cannot be stored for a long term. Thus, when the developer is placed under non-used state for a long time, the characteristics at the initial stage are lowered so that the developer is no longer useful.
Various causes for these drawbacks may be thought of, but, as the result of my studies about the above phenomena, the primary cause was found to be that the distribution in amount of the triboelectric charges of the thus obtained developer was broad and not uniform. In connection with this point, a slightly detailed explanation is given below.
According to the developing methods known in the art, the forces applied on the developer particles are electrostatic attracting force toward latent image and, sometimes electrical forces from outside, image force toward the carriers or the toner carrying member such as a developing sleeve, and adhering or agglomerating forces, and adhesion of the developer particles to the latent image is effected through the total contribution of these forces. Accordingly, the state of triboelectric charge assumed by the developer particles will have important effects on the behaviors of the developer particles during the developing step. On the other hand, the developer containing metal oxide powders such as fine silica powders treated only with an aminosilane compound as described above, cannot maintain in most cases its preferable characteristics for faithful reproduction of latent images, and the distribution in amount of triboelectric charges of the developer becomes very broad. The above mentioned distribution in amount of triboelectric charges is also broad, even when the kind or the amount of the aminosilane compound used for treatment of fine silica powders may be varied, only to give a developer which includes components having smaller triboelectric charges and components having extremely great triboelectric charges after friction with carriers or a toner carrying member such as a developing sleeve. There also exist a slight quantity of components charged to opposite polarity with a greater value. (see Comparative Example 2A and FIG. 3(a), shown below)
In such a developer having a broad distribution of triboelectric charges, the developer components with smaller charge may cause fog or scattering at the edge portion of the latent image, and similar effects may also be incurredby the components charged to opposite polarity. On the other hand, the developer components with greater charge may be affected by increased image force applied from carriers or a toner carrying member such as developing sleeve may not be readily used for developing, whereby lowering of image density or coarsening may undesirably be caused.
The distribution in amount of triboelectric charges of such a developer containing fine silica powder treated only with an aminosilane compound is also susceptible to changes in environmental conditions and will become a distribution unsuitable for development particularly at higher temperature and humidity conditions and lower temperature and humidity conditions. Thus, at higher temperature and humidity, the developer components with smaller charge increase (see Comparative Example 2A, FIG. 3(b)) to cause further markedly fog, lowering of image density, scattering at the edge portion of latent image and lowering of transfer efficiency.
At lower temperature and humidity, the developer components with greater charge increase (see Comparative Example 2A, FIG. 3(c)) to cause more markedly lowering of image density, coarsening and fog and result in increased scattering or drop-off during transfer. Particularly at lower temperature and humidity conditions, this tendency will become further marked by continuous use of the developer, and the initial characteristics of the developer cannot be maintained until the developer becomes no longer useful.
The distribution of triboelectric charges herein mentioned refers to a charged quantity to be measured under the state approximate to the developing system employed. For example, in the two-component developing method, the cascade developing system or the two-component magnetic brush system, charging of the developer particles is effected primarily in the step of contacting with or peeling off from the carrier particle surfaces. Measurement of the distribution in amount of triboelectric charges in such developer systems may be conducted by use of, for example, the method of L. B. Schein et al (J. Appl. Phys. 46, No. 12, P. 5140 (1975)), or the method of R. W. Stover et al (1969 Proc. Ann. Conf. Photo. Sci. Engy SPSE P. 156) and R. B. Levis et al (4th International Conf. on Electrophoto. Adv. Print P. 61 (1981)).